1. Field of the Invention
The invention relates to a method of cutting a mold at high speed, and also to an ultra-high speed milling machine for carrying out the method.
2. Description of the Related Art
Most mechanical parts are manufactured by using molds, and now such molds are indispensable to mass produce high quality mechanical parts at lower costs. Conventionally, it has taken much time and much cost to manufacture a mold. However, as it becomes trendy to produce just a the small number of industrial parts and product life cycles become shorter, there is a strong demand to shorten the period of time for fabricating a mold and to lower the cost of fabrication.
In general, the manufacturing of a mold includes steps, in sequence, of designing, cutting, assembling and finishing, trial and modification. Among these steps, it is presently possible to complete designing in a relatively short period of time because of the development of computer aided (design (CAD) and forming simulation. At present, it takes the longest time to make numerical control programming for manufacturing a mold, and manufacturing of the mold itself.
Mold manufacturing is mostly accomplished by milling by means of an end mill. However, it is impossible to use a large diameter cutting tool for manufacturing of a mold having a complicated shape, except for rough machining. Thus, in most cases, mold manufacturing is accomplished by light-cutting using a small diameter cutting tool. This poses a problem that the depth of cut has to be small, and hence, the machining takes too much time. In order to shorten the time for cutting, it is necessary to increase a rotation speed and hence a feeding speed of the cutting tool. However, a conventional cutting process has many problems for doing so as follows: (a) short lifetime of the cutting tool, (b) difficulty in rotating the tool at a high speed, (c) durability of the bearing, (d) accuracy of a tool holder, (e) mechanism for feeding a cutting tool at a high speed, (f) design of an numerical control (NC) tape, (g) thermal deformation of a processing machine, (h) generation of surface steps of a workpiece due to exchanging tools, and (i) damage to tools due to unexpected increase in depth of cut.
Hereinbelow, the above-mentioned problems are detailed.
If a cutting tool is rotated and fed at a high speed, the lifetime of the tool is made shorter, and thus it will be necessary to frequently exchange tools. As a result, it is impossible to carry out high speed machining.
Though a conventional rotation speed of a tool is smaller than a few thousand rounds per minute (rpm), it is required to increase the rotation speed beyond a few ten thousand rounds per minute.
A conventional ball bearing is able to withstand rotation speed ranging from 10,000 rpm to 50,000 rpm, but is practically unable to withstand a greater rotation speed, because it would have just a short duration o)f life under such rotation speed.
When rotated at a speed greater than a few tens of thousands of rounds per minute, a conventional tool holder would have a problem of loosening and dynamic balance due to centrifugal force in clamps between which a tool is to be clamped.
High speed feeding is indispensable for high efficiency manufacturing, but a conventional ball screw has an upper limit in the range of 20 m/min to 60 m/min. In addition, acceleration and deceleration performances are quite important for high speed feeding, but the driver and/or controller cannot avoid to becoming larger in size in order to make it possible to accelerate or decelerate a ball screw at a short period of time.
It takes much time to make a numerical control tape even now. In particular, for a conventional triaxial control, it takes too much time to make a numerical control tape in order to carry out cutting on a workpiece at a smaller pitch.
If high speed rotation and feeding are carried out, generated heat from a shaft driven at high speed and generated heat during cutting of a workpiece causes the milling machine to be thermally deformed. In particular, such thermal deformation cannot be disregarded in fabricating a mold which has to be accurately dimensioned.
By exchanging tools, a step may be produced on a cutting plane of a workpiece due to dislocation of a newly set tool, elastic deformation of a tool and dispersion in dimension of tools, with the result of the generation of error in surface accuracy.
In general, rough machining and then finish machining have been conventionally carried out to thereby enhance manufacturing efficiency. However, depth of cut often becomes too deep in finishing subsequently to rough machining, and thus the finishing tool is damaged.